Free-gas phase initial pressure



March 3, 1964 J. R. KYTE ETAL RECOVERY or on. FROM WATERED-OUTREszRvoIRs Filed April 1. 1960 IF H/12 I I AIR Inventors Attorney A AIR0 METHANE John R. Kyte Virgil 0. Noumunn By 2...) a. Q;

METHANE FREE-GAS PHASE INITIAL PRESSURE POUNDS PER SQUARE INCH 200 400600 800 I000 I200 I400 I600 I800 2000 mmmmm wD40 mmOm {a mm Ouwm :0

.lnited States 't atent 3,123,134 RECOVERY OF. OIL FROM WATERED-OUT IRESERVOIRS John R. Kyte and Virgil 0. Naumaun, Tulsa, Okla, assignors toJersey Production Research Company, a corporation of Delaware Filed Apr.1, 1960, Ser. No. 19,310 14 Claims. (Cl. 1662) The present inventionrelates to methods .for the recovery of oil from subsurface reservoirsand more particularly relates to an improved process for recoveringadditional oil from previously exploited, watered-out reservoirs. Instill greater particularity, the invention relates to a process foraugmenting recovery from a wateredout reservoir by first establishing ahigh pressure free gas phase in thereservoir and thereafter reducing thepressure on the reservoir.

Experience has shown that techniques utilized in the primaryexploitation of subterranean oil-bearing reservoirs generally permit therecovery of only a small fraction of the total oil initially present insuch reservoirs. In order to stimulate further production after thenatural reservoir energy has largely been expended by primary recoverytechniques, a number of secondary recovery processes have beendeveloped. The most widely used of these is the waterfiooding process.By simply injecting water into an underground oil reservoir through oneor more injection wells and forcing it in the direction of productionwells spaced some distance from the injection wells, oil not recoverableby primary recovery techniques can be produced. An oil reservoir wherein jected water or natural water encroachment has reduced the oilsaturation to a limit near the residual oil satura tion characteristicof the displacement of oil by water in that particular reservoir isgenerally referred to as a watered-out" reservoir. The waterfloodingprocess is considerably more attractive than many other secondaryrecovery processes because the water utilized can ordinarily be obtainedat little cost and need not be recovered from the reservoir in order tomake the process economically feasible.

Watertlooding processes are generally carried out until a high ratio ofwater to oil at the production well makes further exploitationunattractive from an economics standpoint. Studies based in part uponthe fluid content of cores taken from watered-out reservoirs indicatethat from 20 to about 40 percent of the oil initially present may remainin a reservoir at the conclusion of a successful waterflooding project.There are thus many watered-out reservoirs which contain largequantities of potentially recoverable oil. Efforts to produce this oil,however, have largely been unsuccessful. Solvent displacement processesand other methods which might permit additional recovery fromwatered-out reservoirs are generally too expensive to warrant their usein such reservoirs. Less costly processes, those based upon the use ofair or a similar gaseous displacing agent for example, are normally lessefiicient than watertlooding and hence are not effective in watered-outreservoirs. For this reason, ter- 551 exhaust gases from internalcombustion engines and simitiary recovery from waterfiooded reservoirshas not been viewed favorably in the past.

The present invention provides a new and improved process for therecovery of oil from watered-out reservoirs which is considerably moreattractive from the economics standpoint than processes advocated in thepast. In accordance with the invention, it has now been found thatappreciable quantities of oil can be recovered from an oil-bearingreservoir following watertlooding by establishing a high pressure freegas phase in the reservoir and thereafter reducing the reservoirpressure to produce the oil contained therein. The expression free-gasphase halted and the well is permitted to flow.

3,123,134 Patented Mar. 3, 1964 ice as used herein refers to gas notdissolved in liquids present in the system. The use of this processgenerally permits the recovery of from about 30 to about 50 percent ofthe oil remaining in the affected portion of the reservoir afterwaterflooding and, where repeated pressuring and pressure reductionsteps are utilized, may permit even more significant results. The costof the process is low and hence it can be applied to many reservoirsfrom which additional oil cannot be economically recovered by any otherknown method.

The process of the invention is normally carried out in a watered-outreservoir containing one or more injection wells and one or moreproduction wells spaced at some distance from the injection wells. Gasis injected into the injection wells in quantities and at a pressuresufficient to establish the necessary free gas phase. In mostreservoirs, particularly those in which a water drive contributed toprimary recovery, the production wells may be closed off prior to theintroduction of gas at the injection wells. In some small reservoirs,however, it may be preferable to withdraw fluids from the productionwells during the establishment of the free-gas phase. During anoperation of the latter type, the production wells may be subjected to asuitable back pressure while gas is being introduced at the injectionwell or may instead be allowed to flow free initially and may thereafterbe closed off as additional gas is injected to build up the reservoirpressure to the desired level. Fol-lowing the establishment of the highpressure free-gas phase in this manner, the injection wells are closed01? and the production wells are allowed to flow. If desired, backpressure may be maintained at the production wells and gradually reducedto control the flow. Substantial quantities of oil, together with gasand water, flow into the production wells and are produced. Aftersubstantially no quantities and at a pressure sufiicient to establishthe requisite free-gas phase. Thereafter, gas injection is Oil, gas andwater are produced until the oil production rate declines to anuneconomical level. Thereafter, gas may again be injected into thereservoir and the cycle repeated.

A number of gases may be employed in carrying out the process of theinvention, the principal requirement for the gas selected being that itnot condense under reservoir conditions of temperature and pressure.Gases whichrnay be utilized include air, natural gas, fiue gases,

l-ar gases available at low cost. Air and natural gas are preferredbecause of their ready availability and will be employed most frequentlyin practicing the invention.

The pressure of the free-gas phase established in the reservoir willgenerally range from about pounds per square inch to about 3,000 poundsper square inch. It has been found that the quantity of oil recovered inthe process of the invention increases with increasing pressure but thatthe greater recovery realized at extremely high pressures generally doesnot warrant the higher compression costs incurred. For this reason, itis generally preferred to employ gas pressures ranging from about 400 toabout 1,500 pounds per square inch. The pressure utilized will, ofcourse, depend upon the pressure in the reservoir at the onset of theoperation. Sufficient pressure to permit injection of the gas must beused and hence higher gas pressures may be utilized in after reducingthe pressure.

high pressure reservoirs than would otherwise normally be used.

The amount of gas injected into the reservoir to establish the free-gasphase will generally range between about one percent and abouttwenty-five percent of the reservoir pore volume, determined at thereservoir temperature and the pressure of the free-gas phase to beestablished in the reservoir. The injection of gas in excess of abouttwentyfive percent of the reservoir pore volume has little or no effectupon the oil recovery obtained and in many cases may increase the costof the operation to the point where it becomes economicallyunattractive. The use of from about 1 percent to about 15 percent ofgas, based upon reservoir pore volume, is preferable in most instances.

The exact nature and objects of the process of the invention can best beunderstood by considering the results obtained in experimental workduring which oil recovered from the core during this pressure reductionstep. The final oil content of the core was 14.7 percent of the porevolume. This represents a forty percent reduction in the oil content ofthe core following waterflooding.

Following the initial test described above. additional tests werecarried out in similar manner using air, nitrogen, flue gas and methane.Gas pressures were varied between about 200 pounds per square inch andabout 1630 pounds per square inch. Tests were carried out with Soltrol,a refined white oil having a. viscosity of 1.4 centipoises, as well aswith the Loudon crude oil. A 45-inch Weller sandstone core was employedin some of the tests. The amount of gas injected ranged from 9 to 16percent of the pore volume. The results obtained in this first series ofexperiments are set forth in Table I below.

Table I EFFECT OF GAS COMPOSITION ON OIL RECOVERY FROM WATERED-OUT CORESOil Con- Oil (on- Oil lte- Oil Con- Oil Re- Gas Intent After" tout.After covered by tent After covered by Run Oil Gas Gas Prtsjoetiul, lor-Waturflootl, (his Drive, (his Drive, Pressure Pressure sure, psi. contloro l'tl't'tnt Percent Percent Redue- Reduc- Volunie Pore Vol lorv \ol-Porn Voltion, Pertion, Perutuo time urne cont Pore cent; Pore VolumeVolume Methane" 1,100 12 24. 3 34. 2 0. l 14. 7 9. 5 Nitrogen" 1. 630 12La. 2 22. 0 0. 2 13. 1 8.9 Air 1,000 11. 5 28.0 97.3 0.2 19.1 8.7 FlueGas" S 15 34. 1 33. 9 0. 2 24. 5 9. 4 Methane. 930 16 32. 4 32.3 0.123.1 9. 2 -tl0 470 12 35.3 35. 1 0.2 22.3 12.8 Air 500 11 33. 4 33. 2l). 2 21. 9 11. 3 .Methaneu 940 10 33. 'r' 33. 6 0. 1 19. 6 14. 0 Air1,000 11 33.9 33. 7 0.2 21.0 12. 7 Methane" 1.100 9 33. 2 32. 7 0. 518.0 13. 8 Air l, 170 10 33. 0 32. 9 0. 1 18. 1 15. 0 Methane 210 10 33.9 6 0. 3 24. t) 9. 6 Air 200 .10 33. 9 33. 7 0. 2 26. 7 7. 1

was recovered from watered-out cores by first establishing highpressure,free-gas phases in the cores and there- The attached drawingwhich shows the relationship between the initial pressure of thefree-gas phase established and the oil recovered upon reduction of thepressure further illustrates the invention.

EXAMPLE I In the first of a series of experiments. a 13-inch core ofWeller sandstone measuring 2 inches in diameter was prepared by placingit in a section of pipe and using a low melting metal alloy whichexpands upon cooling to seal it in place. The pipe was provided with aninlet and a longitudinally opposed outlet for injecting and withdrawingsfluids. The core was then saturated with about 65 volume percent oilandabout volume percent connate water. In this initial test, crude oilobtained from the London field in Illinois was used. The saturated corewas thereafter flushed with water until all of the oil displaceable bywater had been recovered from it. The oil content of the core was thendetermined. It was found that the waterfiooding'reduced the oil contentof the core to 24.3% of the pore volume.

Following the waterfiooding as described above, methane was injectedinto one end of the watered-out core at a pressure of about 1100 poundsper square inch. About 12 percent methane, based on core pore volume andmeasured at ambient temperature and the selected pressure was used. Thefluids displaced by the injected gas were recovered at the opposite endof the core and analyzed. it was found that thus driving gas through thewatered-out core displaced very little of the oil contained in the core,only about 0.1 percent of a pore volume.

After a high pressure, free gas phase had been established in the coreas described above, the pressure upon the core was reduced and thefluids produced with the escaping gas were collected and analyzed. Asignificant quantity of oil, about 9.5 percent of the pore volume, was

From the above table it can be seen that driving gas through thewatered-out cores did not permit the recovery of significant quantitiesof oil. This confirms earlier experience and is not unexpected. since itis known that waterfiooding in general is considerably more effectivethan gas drive as a method for displacing oil from subsurfacereservoirs. The data show, however, that the formation of a free-gasphase followed by a pressure reduction step resulted in the recovery offrom about 20 percent to about percent of the oil present in the coresfollowing waterfiooding. The process of the invention thus permits therecovery of significant quantities of oil which cannot be recovered byordinary waterfiooding and gas driving techniques.

The data set forth above also demonstrate that the process of theinvention may be carried out with a variety of gases. Air, nitrogen,methane and flue gas all permitted the recovery of substantialquantities of oil. In most cases the results obtained with methane andline gas were only slightly better than those obtained with air andnitrogen, indicating that gas solubility in the oil may be of somesignificance but that the effect due to solubility is small. The factthat substantial recoveries were obtained with both the Loudon crude oiland the refined white oil indicates that the process is not greatlydependent upon the properties of the oil in the reservoir andmay beapplied to reservoirs containing oils which vary considerably withrespect to their composition, vis cosity, gravity and othercharacteristics.

EXAMPLE XI A second series of experiments were carried out to determinethe effect of pressure upon the process of the in vention. The Weilersandstone cores employed in the previous tests were cleaned, dried andresaturatedwith connate water and the refined white oil referred toearlier and then wa-terfiooded until no more oil could be displaced. Theoil content of the watered-out core was determined for each run and wasfound to be about 33 to 34 percent in terms of core pore volume. From 9to 12 percent methane or air, measured at the pressure selected andbasedupon core pore volume, was then injected into the core during each run.The pressures utilized ranged from 190 pounds per square inch to 1700 apressure of 1150 pounds per square inch. Air was employed as theinjection gas and Soltrol, a refined white oil, was used as the oil.iRecovery during the pressure reduction step of the process ranged from10.8 percent to 15.0 percent, based upon pore volume. The data obtainedare shown in Table 111.

Table III EFFECT OF GAS VOLUME 0N OIL RECOVERY Oil Con- Oil Con- 01] Re-011 Con- Oil Re- Gus Intent Alter tent After covered by tent Altercovered by Gus Presected. Per- Waterflood, Gas Drive, Gas Drive,Pressure Pressure Run 011 Gas sure, p.s.i. cent Pore Percent PercentPercent Reduc, Reduc- Volume Pore Vol- Porn 701- Pore Voltion, Pertlon,Perurns ume ume cent Pore oent'Pore Volume Volume l 1,150 4.6 33.9 33.9o 19.6 14 a 1,150 33.3 33.2 0.1 18.2 0 1,150 12 33.4 33.0 0.4 19.9 13 1I, 150 688 33.4 32.6 0.8 17.7 14.9 1,150 711 33.5 31.4 2.1 20.6 10 8pounds per square inch. The fluids displaced from the cores werecollected and analyzed. The oil recovered as a result or" the gas driveranged from 0.1 to 0.5 percent, based upon core pore volume. The coreswere then depressurized and the fluids produced with the gas werecoilected and measured. The oil recovery during the pressure reductionstage of the process ranged from 8.3 percent of the pore volume at aninitial pressure of 190 pounds per square inch to 16.4 percent of thepore volume at an initial pressure of 1700 pounds per square inch. Theresults of this second series of tests are set forth in greater detailin Table 11 below.

Inspection of the data set forth in Table III shows that increasing thevolume of gas injected above 4.6 percent of the pore volume had littleelfect upon the oil recovery obtained. Substantially the same recoverywas obtained by pressure reduction after 4.6 percent gas, based uponpore volume, had been injected as was obtained following the injectionof much larger quantities of gas. In an actual reservoir, substantiallyless than 4.6 percent gas will normally be elfective. The injected gasdoes not sweep the entire reservoir and hence the etfective gasconcentration in the swept area is higher than the volume of injectedgas would indicate. For this reason, it is pre- T able II EFFECT OF GASPRESSURE ON OIL RECOVERY FROM WATERED-OUT CORES O11 Con- O11 Con- 011Re- 011 Con- 011 Re- Gss Intent Alter tent Alter covered by tent Aitercovered by Gas Presjected Per- Waterflood, Gas Drive, Gas Drive,Pressure Pressure Run Oil Gas sure, p.s.i. cent are Percent PercentPercent Reduc- Reduc- Volume Pore Vol- Pore Vol- Pore Voltion, Perflon,Perume ume ume cent Pore cent Pore Volume Volume Methane 190 9 33. 533.3 0. 2 25. 0 8. 3 210 10 33. 9 33. 6 0. 3 24. 0 9. 6 450 9 33. 8 33.7 0. 1 21. 2 12 5 470 12 35. 3 35.1 0. 2 22. 3 12. 8 900 9 33. 7 33. 50. 2 20. 2 13. 3 900 9 33. 3 33. 2 0. 1 10. 2 14. 0 040 9 34. 3 34.1 0.2 20. 9 13. 2 940 10 33. 7 33. 6 0. 1 19. 6 14.0 1,100 9 33.2 32. 7 0.518.9 13.8 1, 700 9 33. 5 33. 3 0. 2 16. 9 16. 4 200 10 33. 9 33. 7 0. 226. t1 7. 1 500 11 33.4 33.2 0.2 21.9 11.3 1,000 11 33.9 33.7 0.2 21.0127 1.500 10 33.3 33.2 0. 1 18.2 15.0 1, 500 12 34.2 9 0.3 18.2 15.7 1,500 10 34. 0 33.9 0. 1 17. T 16. 2 I, 500 10 34. 0 33. 9 0.1 21. 2 12. 7

The data in Table 11 show that the oil recovery from a watered-out coreincreases with increases in the initial pressure of the free-gas stageestablished in the core prior to the pressure reduction step. These dataare presented graphically in the drawing, from which it can be seen thatthe recovery increases with pressure rapidly up to about 400 pounds persquare inch and that the rate of increase is slower thereafter. Becausethe cost of compressing gas becomes prohibitively high at extremely highpressures, it is generally preferred to carry out the process of theinvention at pressures between about 400 pounds per square inch andabout 1500 pounds per square inch.

EXAMPLE 111 Further tests were carried out to determine the effect ofthe volume of gas injected upon oil recovery. The procedure employed wasessentially the same as in the earlier tests except that the injectedgas volumes ranged from about 4.6 percent up to about 722 percent, basedupon core pore volume. All the tests were carried out at ferred to carryout the process of the invention by injecting only from about onepercent to about twenty-five percent gas, based upon pore volume, intothe reservoir. The fact that the injection of 722 percent gas resultedin the recovery of only 2.1 percent oil by gas drive in the aboveexperiments clearly demonstraes that conventional gas drive processesvapplied to non-watered-out reservoirs are not effective after awaterflooding process has been carried out.

EXAMPLE 1V inlet. It was found that substantial quantities of oil wereproduced from the cores in this manner. The results of these tests areset forth in Table IV.

The core was repressured to 1050 pounds per square inch and again thepressure was reduced. In this second cycle, 4.5% oil was recovered. Athird cycle carried out in the Table IV EFFECT OF PRESSURE REDUCTION ONRECOVERY FROM SINGLE WELL Oil Con- Oll Con- Oil 120- Oil Con- Oil Re-Gas Intent After tent After covered by tent After covered by I GasPresjt-cted, Ier- Waterflood, Gas Drive, Gas Drive, Pressure PressureRun Oil Gas sure, p.s.i. ccnt Pore Percent Percent Percent Retluc-Reduc- Volume Pore Vol- Pore Vol- Port: Vollion, lertion, Perumc untoulnu cent Pore cent Pore Volume Volume A Soltrnl Air 1,150 4. 3 34.134.1 27. Z 6. 9 B "do .do l. 150 4. 33 8 33. 7 0. l 27. 6 6 l c (in rln1,150 12 33 .9 33.7 0.2 21.9 11.8

same manner yielded 3.5% oil. additional recovery was obtained.

A second set of tests using methane and Wyrol confirmed the resultsobtained with the heavy oil. When In a fourth cycle, no

repeated cycles were used with a methane-Soltrol system;

however, no additional recovery resulted following the initial pressurereduction step. The results obtained in the two sets of tests usingWyrol appear in Table V.

Table V EFFECT OF REPEATED CYCLES ON RECOVERY FROM WATERED-OUI CORES OilCon- Oil Con- Oil Rc Oil Con- Oil Re- Gns Intcnt After tent Aftercovered by tent After covered by Gas Presjccted, Per- Watcrfloml, GusDrive, Gas Drive. Pressure Pressure Run Oil Gas sure, p.s.i. cent PorePercent Percent Percent Reduc- Reduc- Volume Pore Vol- Pore Vol- PoreVoltion, Pertion, Pertime time urne cent Pore cent Pore Volume Volume AWyrol Methane- 1, 050 12 37.4 30.9 0.5 30. 5 64 Second Cycle Do do 1.05012 26.0 4 5 Third Cycle Do do 1,050 12 22.5 3 5 Fourth Cycle Do .do...1,050 12 22.5 0 13 Wyrol Mcthanc. 1,050 12 36.2 36.1 0.1 28.2 7. 9

Second Cycle Do d0 1.050 12 25.2 3 0 Third Cycle Do l0 1,050 12 25.2 0

pected. Recovery by means of a single well provides an attractive methodfor securing additional oil from many small watered-out fields.

EXAMPLE V Tests similar to those described earlier were carried out todetermine the elfect of repeated repressuring and pressure reductioncycles in the process of the invention. The

first of these utilized methane and Wyrol, a heavy, refined white oilhaving a viscosity of about centipoises. The core employed was firstsaturated with the Wyrol and connate water. After waterfiooding until nomore oil could be displaced, a free gas phase was established in it willbe noted that repeated gas injection and pressure reduction stepspermitted the recovery of substantially more of the heavy viscous oilthan could be recovered in a single cycle. There are many watered-outreservoirs which contain such heavy oils and hence repeated cycles canoften be employed to advantage. In reservoirs containing less viscousoils similar to Soltrol, repeated cycles have little effect.

It will be recognized that several modifications may be made in theprocess described above without departing from the scope of theinvention. In many cases, for example, it may be desirable to inject gasinto a wateredout reservoir through one well and to thereafter reducethe reservoir pressure and produce reservoir fluids through both thatwell and adjacent wells. This and other similar modifications will bereadily apparent to those skilled in the art.

What is claimed is:

1. An improved process ,for recovering oil from a watered-out oilbearing reservoir underlying the earths surface which comprisesinjecting a noncondensing gas into said reservoir through at least onewell penetrating said reservoir, said gas being injected at a pressurewithin the range between about 100 and about 3,000 lbs. per square inchand in a quantity sufficient to establish a free-gas phase therein;discontinuing the injection of said gas; reducing the'pressure on saidreservoir; and producing reservoir fluids through at least one wellpenetrating said reser- V011.

2. A process as defined by claim 1 wherein said gas is injected at apressure of from about 400 to about 1,500 pounds per square inch.

3. A process as defined by claim 1 wherein said non- I condensing gasinjected into said reservoir is air.

4. A process is defined by claim 1 wherein said noncondensing gasinjected into said reservoir is natural gas.

5. A process as defined by claim 1 wherein a noncondensing gas isreinjected into said reservoir and the pressure thereon is again reducedafter the rate at which fluids are produced from the reservoir declinesto an undesirably low level.

6. An improved process for augmenting production from a well penetratinga watered-out oil-bearing reservoir which comprises injecting from about1 to about 25 percent, based upon reservoir pore volume and measured atthe injection temperature and pressure, of a noncondensing gas throughsaid well into said reservoir at a pressure of from about 100 to about3000 pounds per square inch to establish a free-gas phase therein,discontinuing the injection of said gas, reducing the pressure on saidwell, and recovering oil, gas and water flowing into said well from saidreservoir.

7. A process as defined by claim 6 wherein an exhaust gas from aninternal combustion engine is injected into said reservoir as saidnoncondensing gas.

8. An improved process for recovering oil from a watered-out subsurfaceoil-bearing reservoir following a waterfiooding operation whichcomprises injecting from about 1 percent to about 25 percent, based uponreservoir pore volume and measured at the injection temperature andpressure, of a noncondensing gas into said reservoir through at leastone injection well at an injection pressure of from about 100 to about3000 pounds per square inch whereby a free-gas phase is established insaid reservoir; discontinuing the injection of said gas; reducingpressure onsaid reservoir at at least one production well spaced fromsaid injection well; and withdrawing oil, water and gas flowing intosaid production well from said reservoir.

9. A process as defined by claim 8 wherein irom about 1 percent to aboutpercent air, based upon reservo r pore volume and measured at theinjection pressure, 18 injected into said reservoir as said noncondensmggas.

10. A process as defined by claim 8 wherein from about 1 percent toabout 15 percent natural gas, based upon reservoir pore volume andmeasured at the injection pressure, is injected into said reservoir assaid noncondensing gas.

11. A process as defined by claim 8 including the additional steps ofdiscontinuing the withdrawal of fluids from said reservoir aftersubstantially no more oil can be recovered, injecting a noncondensinggas into said reservoir through said injection well at a pressure offrom. about 100 to about 3000 pounds per square inch, reducing thepressure on said reservoir at said production well, and recoveringadditional oil from said production well.

12. A process as defined by claim 8 wherein, following injection of saidgas at said injection well, the pressure on said reservoir is reduced atboth said injection well and said production well and fluids arewithdrawn from said reservoir through both said injection well and saidproduction well.

13. A process is defined by claim 8 wherein said gas injected is a fluegas.

14. An improved process for recovering oil from a subterraneanoil-bearing reservoir which comprises injecting water into saidreservoir through at least one injection well, recovering fiuids fromsaid reservoir through at least one production well spaced from saidinjection well, continuing said injection of water and recovery offluids until said reservoir is watered-out, thereafter injecting fromabout 1 to about 25 percent, based upon reservoir pore volume andmeasured at the injection temperature and pressure, of air into saidreservoir through said injection well at a pressure of from about 400 toabout 1500 pounds per square inch to establish a freegas phase whilemaintaining a back pressure at said production well, discontinuing theinjection of gas at said injection well, reducing the pressure on saidreservoir at said production well, and recovering oil from saidreservoir through said production well.

References Cited in the file of this patent UNITED STATES PATENTS1,067,868 Dunn July 22, 1913 2,669,307 Mulholland et al Feb. 16, 19542,875,831 Martin et ai. Mar. 3, 1959 OTHER REFERENCES PetroleumProduction Engineering,- Exploitation, by Uren, 2nd edition, publishedby McGraw-Hill Book Co. of New York in 1939, pages 423 to 426.

Dickey et al.: Article in Secondary Recovery of Oil 0' in the UnitedStates, see. ed. pub. 1950 by American Petroleum Institute, W. 50th St.,New York, N.Y. pages 444 to 462.

Talash et al.: Article in the Petroleum Engineer, Sept. 1957, pages 3-27to B30.

1. AN IMPROVED PROCESS FOR RECOVERING OIL FROM A WATERED-OUT OIL BEARINGRESERVOIR UNDERLYING THE EARTH''S SURFACE WHICH COMPRISES INJECTING ANONCONDENSING GAS INTO SAID RESERVOIR THROUGH AT LEAST ONE WELLPENETRATING SAID RESERVOIR, SAID GAS BEING INJECTED AT A PRESSURE WITHINTHE RANGE BETWEEN ABOUT 100 AND ABOUT 3,000 LBS. PER SQUARE INCH AND INA QUANTITY SUFFICIENT TO ESTABLISH A FREE-GAS PHASE THEREIN;DISCONTINUING THE INJECTION OF SAID GAS; RE-